# Category:949 Other Aspects of Traffic

## 949.2 Prevailing Speed Determination

Prevailing speed determination is the method used by the department as criteria for speed limit determination in incorporated and unincorporated areas.

Three separate sets of data shall be utilized to establish the prevailing speed. These are:

• 85th percentile speed; used only when ADT is 500 or greater;
• Upper limit of the ten MPH pace; used only when ADT is 500 or more; and
• Average test run speed.

The prevailing speed of free flowing traffic shall be determined and the average of the actual value established for the three criteria above.

Based on the assumption that the majority of the drivers, approximately 85 percent, will drive at a speed that they judge to be safe and proper, the selected speed limit in 5 mph increments will not exceed the established prevailing speed by more than 3 mph.

When justified by further investigation, the selected speed limit may be less than the established prevailing speed. Justification for reduction will be determined by a study of the following conditions.

1. Accident Rate
3. Traffic Control Devices
4. Length of Section
5. Pedestrian Traffic
6. Parking

### 949.2.1 Accident Rate (AR)

If the accident rate, based on reportable accidents within the proposed zone, is 50 percent higher than the statewide average accident rate (SWAR) for the same classification of highways, prevailing speed may be reduced by 5 percent. If the accident rate is more than twice the statewide rate for this classification, the prevailing speed may be reduced by 10 percent. Minor accidents, not reportable under state statutes, are not to be considered since the reduction in speed will not normally reduce the number of accidents but may reduce the severity of those accidents that occur. Current information regarding accident rates can be found in TMS.

##### Table 949.2.1 Prevailing Speed Reduction
Reduction Condition Pevailing Speed Reduction
AR > 1.5 SWAR 5%
AR > 2.0 SWAR 10%

The effect of driveway entrances will be determined by using a Driveway Conflict Number. For this purpose, a private or field entrance shall have a Driveway Conflict Number of 1. Minor commercial entrances shall have a number of 5. Major commercial entrances, shopping centers or industrial plant entrances that generate relatively high volumes of traffic, and public streets shall have a number of 10. If the total Driveway Conflict Number exceeds a rate of 40 per mile through the proposed zone, the prevailing speed may be reduced by 5 percent. If the total number exceeds a rate of 60 per mile, the prevailing speed may be reduced by 10 percent. However, before a reduction can be made due to the Driveway Conflict Number, the Poisson Curve must be used to determine if the accident reduction is statistically significant.

Figure 949.2.2

The test is as follows:

a) Consult Fig. 949.2.2, using the current number of accidents for the last year.

b) Determine the graph’s percent value from the Poisson Curve.

c) Compute accident rate for last year.

$\mbox{Accident Rate}=\frac{\mbox{no. of crashes x 100,000,000}} {\mbox{365 x ADT x length in miles}}$

d) Obtain Statewide Accident Rate (SWAR). The accident data is obtained from the Transportation Management System (TMS).

e) Calculate percent reduction between accident rate and statewide accident rate by:

$\mbox{Percent Reduction}={100}\frac{\mbox{AR-SWAR}} {\mbox{AR}}$

f) Compare % reduction with value obtained in step b.

g) If % reduction equals or exceeds value obtained in step b, the Driveway Conflict Number may be used as the action would be statistically significant.

Example:

8.0-mile section of roadway, ADT = 3,100, 30 accidents within last year.
a & b). Value obtained from Fig. 949.2.2 = 30 %
c).$\mbox{AR}=\mbox{331.4}=\frac{\mbox{30(100,000,000)}} {\mbox{(365)(3,100)(8.0)}}$
d). SWAR = 242.04
e).$\mbox{26.9 percent reduction}={100}\frac{\mbox{331.4-242.04}} {\mbox{331.4}}$
f). Compare % reduction with value.
26.9 < 30
Since 26.9 is less than 30, the Driveway Conflict Number cannot be considered.

### 949.2.3 Traffic Control Devices

Sections of roadway consisting of a curvy or series of curves and turns and other geometric problems should be signed with the appropriate warning signs where it is customary to use these signs. Speed limit signs would not be considered where a warning sign is needed to advise the motorist of an approaching condition.

### 949.2.4 Length of Section

Enter the distance.

### 949.2.5 Pedestrian Traffic

Where sidewalks are not provided adjacent to the route and a total pedestrian traffic along the route exceeds 10 per hour for 3 hours of any 8-hour period, the prevailing speed may be reduced by 5 percent. Pedestrians crossing the route may be counted if the point of crossing is not protected by some other form of traffic control device.

### 949.2.6 Parking

Where parking is permitted adjacent to the traffic lane, the prevailing speed may be reduced by 5 percent. The prevailing speed determined by the 85th percentile speed, the pace speed and the average test run shall not be reduced by more than 10 mph by using Steps a through f of 949.2.2. All reductions shall be in 5 mph increments.

### 949.2.7 Procedures for Obtaining Prevailing Speed Data

Spot speed studies can be made with any speed measurement device such as radar, count cards and inductive loops that determine the 85th percentile speed and the upper limit of the 10 mph pace. The 85th percentile speed is defined as the speed at or below which 85 percent of the vehicles are traveling. The 10 mph pace is defined as the 10 mph range containing the most vehicles.

Spot speed studies should be made as close to the center of the proposed zone as is practical. If the zone is lengthier than one mile, studies should be made at a minimum of two locations. If the difference in data between the two locations is minor, the higher value should be used. If the difference in data is over 5 mph, consideration should be given to designating separate zones.

Exercise care to collect the data in a manner and at times that indicate normal conditions. Normal conditions will be assumed to prevail under good weather conditions on dry pavement, following morning rush hours and prior to the evening rush hours, on any day of the week except Saturdays, Sundays or holidays. Observations should not be made immediately following a significant event or during a period of greater than normal police enforcement. Every effort should be made to disguise or conceal the fact that speeds are being reported.

Speeds should be observed for a least 100 passenger cars and pick-up trucks. Trucks over 4 tons will not be included in the data for determining a revised limit.

Average test run speeds shall be determined on the basis of at least two runs in each direction over the length of the proposed zone. The prime consideration in these test runs is to determine a maximum permissible speed. Speeds are to be recorded at 0.1-mile intervals. While making the test run, the driver will try to “float” in the traffic stream, passing as many vehicles as pass the test car.

### 949.2.8 Additional Guidelines for Establishing Speed Limits

Normally, isolated curves and turns, areas of restricted sight distance, no passing zones, etc. will not be considered as areas for lowering speed limits. The accident record on such highways will be the principle criterion. Critical speed signs shall be used in connection with the appropriate warning signs at these locations.

Except on divided highways, different speeds for traffic in opposite directions cannot be justified.

Speed limits shall begin at a point on or as near in advanced of the point as possible where the speed limit is warranted and shall end at the point or as near the point as possible where the speed limit is not warranted.

## 949.3 Local Exchange of Transportation Services (LETS)

Article under construction.

## Articles in "949 Other Aspects of Traffic"

The following 3 pages are in this category, out of 3 total.